1. Field of the Invention
The invention relates to an electrically shielded module carrier for receiving electric and/or electronic modules, the carrier including a cover element and a base element, with at least one mounting space, which is divided into a front section and a rear section by an intermediate wall card. Holding elements for inserted flat modules are provided laterally in each of these sections.
2. Description of the Related Art
Module carriers serve to receive different electric and electronic units. In many areas of electrical engineering, in particular in the area of communication technology, the problem arises that in the module carrier the mounting space for electronic units has to be protected against parasitic radiation, while electric components, such as fans, power supply units and cabling, also have to be housed in module carriers. If these electrical units themselves are not to be considered to be a source of interference, the manner in which they are housed in the module carrier requires comparatively fewer shielding measures.
Depending on the type of field to be shielded, different models and designs of shielding housing are known in electrical engineering.
To protect against electrostatic effects and slowly variable fields, it is known that the entire housing of an electrical device can be designed in the manner of a Faraday cage. Plate components made of steel, aluminum or special alloys are generally used for shielding devices, assembled using mechanical and electrical connections to form a conductive shielding jacket. Depending on the technical design of the connections, the mounting space to be protected is ideally field-free.
In the event of incident and emergent electromagnetic waves, the shield effect is determined by reflection and transmission at material interfaces and by absorption in the shielding wall. In order to satisfy the requirements of a perfect electromagnetic shield, the housing components have to be connected by a plurality of contact points, each with the lowest possible contact resistance. Structurally this means all-round contact of the joint areas with closely spaced contact points. Such all-round contact, which can be achieved by spot welding, screws, rivets, screened springs, contact lugs or flexible sheathed electric cables, requires corresponding expenditure.
A further problem results from the demand for increasing integration and component density. It has to be possible to insert the assembled printed circuit boards from the front and from the rear into the module frames, i.e. the covers at the front and rear of the module carrier, which are incorporated in the shield, have to be removable. With this model of module carrier the electrical connection of the printed circuit boards is effected by a connector card arranged across the direction of insertion. This connector card, hereafter referred to as the intermediate wall card, is also referred to in the literature as the midplane or double-sided backplane. It is generally arranged in the center of the module carrier and is provided with plug-in connectors on both sides. The printed circuit boards can be pushed into these connectors. A shared bus provides an electrical connection between the plug-in modules. The intermediate wall card also often contains active components and it must be accessible or such that it can be disassembled for service operations.
Given the structure of the module carrier, this means that the mounting space must be designed as a space with a high level of shielding both in the front and the rear sections. On the other hand the intermediate wall card must also be accessible and as simple as possible to mount during assembly. If, for reasons relating to shielding, the intermediate wall card also has to be integrated into the shield, the latter must make reliable contact with the shielded housing components.
From an economic point of view shielding costs should where possible be tailored to the respective shielding requirements in the module carrier. In other words, where the volume to be mounted is subject to less stringent requirements with regard to EMC, it should also be possible to reduce costs.
It should also be possible to mass-produce the module carrier, where possible with standard production tools.
A module frame with a central backplane is known from EP 0926937. The backplane is mounted on a backplane carrier. To mount the backplane there are two knobs in the central area of the module frame, which serve as axes of rotation and which are arranged both in the cover element and the base element. These knobs can be inserted by swiveling the backplane into cutouts on the backplane carrier, whereby the backplane can be positioned in the central area of the module frame. Production of the module frame is comparatively expensive, as the backplane carrier represents an additional component with additional production costs.